Films used in meat packaging applications are generally multilayer compositions having at least one layer capable of providing the film with good oxygen barrier properties. Frequently, the barrier layer of these films comprises a vinylidene chloride - vinyl chloride copolymer, commonly designated as polyvinylidene chloride or PVDC. Although PVDC layers generally provide films with relatively low rates of gas transmission, over a period of time PVDC copolymers tend to degrade, forming degradation products which include chromophores. The presence of chromophores accounts for the yellow tint often observed in PVDC-containing films.
A pronounced yellow tint is generally unacceptable in films used to package meat products, particularly fresh red meat cuts. Films so tinted tend to impart a yellow appearance to meat fat which customers frequently associate with undesirable product aging or spoilage.
Exposure to elevated temperatures and/or irradiation generally accelerates the rate of PVDC degradation, thereby accelerating the rate of chromophore production and intensifying the degree of film yellowing. However, irradiation of PVDC-containing films often improves properties such as the puncture resistance, tensile strength and toughness thereof. Accordingly, in the production of PVDC-containing films for meat packaging applications, manufacturers must frequently weigh the undesirability of film discoloration against the property benefits obtained by irradiation. Additionally, undesirable film yellowing may result from polymeric resins reaching degradation inducing temperatures during film extrusion. When this occurs it is common practice to stop production to remove the yellowed resin from a system. Thus, in terms of system shut-down time and raw material loss, this type of thermally induced PVDC degradation can have an economically disadvantageous impact on film production.
Prior to this invention, several attempts were made to produce a PVDC-containing film suitable for meat packaging applications by the incorporation of one or more acceptable pigments into same. That is, only certain film colors are considered aesthetically acceptable in meat packaging with PVDC-containing films. These attempts resulted in films wherein discoloration was either ineffectively masked or wherein the pigmented films took on an undesirable tint following irradiation and/or elevated temperature exposure. Masking has been found to be particularly difficult since throughout the period of film storage the color being masked is continuously intensifying.
Accordingly, it is an object of this invention to provide a transparent heat-shrinkable, vinylidene chloride-vinyl chloride copolymer-containing film which has an aesthetically acceptable appearance for packaging fresh and processed meats. It is a further object of this invention to provide a transparent heat shrinkable, vinylidene chloride-vinyl chloride copolymer-containing film which upon exposure to elevated temperature and/or irradiation retains an aesthetically acceptable appearance for meat packaging applications, preferably, over extended storage periods.